Low bulk density, fluffy powders generally are difficult to compact in traditional equipment, without additives, due to difficulties in feeding the powder into the compaction zone of the equipment.
Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin clay in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin clay is used to extend fiber and reduce cost and to improve opacity, brightness and other desirable characteristics of the paper product.
Calcined kaolin clay is a particular type of kaolin and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating beneficiated kaolin clay at temperatures of at least 550° C. The calcination step dehydroxylates and converts the kaolin to a noncrystalline aluminosilicate phase. The term “dehydroxylates” refers to the removal of structural hydroxide groups from the kaolin as water vapor. Calcined kaolin has improved light scattering characteristics (as compared to the non-calcined kaolin) and, therefore, contributes a high degree of opacity to the coated paper.
Fanselow et al. U.S. Pat. No. 3,586,523 relates to calcined kaolin clay.
Calcined kaolin clay pigments (such as the product marketed under the trademark KAOCAL by Thiele Kaolin Company of Sandersville, Ga.) are widely used in the paper industry. Commonly, calcined kaolin clay is blended with hydrous kaolin clay in the paper mills prior to the manufacture of a finished paper product. These blends are typically comprised of about 10 to about 30 parts by weight calcined kaolin and about 70 to about 90 parts by weight hydrous kaolin.
During processing, the calcined clay is often pulverized in a high energy impact mill and then air-classified to remove abrasive particles larger than 325 mesh, as these particles tend to cause scratching problems during coating of a paper. The resulting pulverized calcined clay product is fluffy and has a low bulk density when compared to a spray dried hydrous kaolin clay. For example, the bulk density of calcined kaolin is often about 10 to about 15 pounds per cubic foot, whereas the bulk density of spray dried hydrous kaolin is often about 40 to about 55 pounds per cubic foot.
The fluffy, low bulk density calcined clay product can be difficult to handle with conventional bulk handling systems. Therefore, the product is typically shipped dry in bags, rail freight cars or as an optimally dispersed slurry, typically at about 50% solids. In addition, because of low bulk density, dry calcined clay will commonly require either larger bags or larger volume rail freight cars when compared to a spray dried hydrous clay, which results in higher shipping costs for a dry calcined clay.
In the industry, efforts have been made to improve the bulk density, dusting and powder flow characteristics of calcined clay. Examples of such efforts are shown in Suitch et al. U.S. Pat. Nos. 5,074,475 and 5,129,953; Cook et al. U.S. Pat. Nos. 4,561,597 and 4,593,860; Dunaway et al. U.S. Pat. No. 5,364,579; Crumbley et al. U.S. Pat. No. 5,328,506; and Munsterman et al. International Patent Publication No. WO 97/42268.
A process for the manufacture of high bulk density agglomerates from a mixture of calcined kaolin clay, hydrous kaolin clay and water is disclosed in Maxwell & Malla U.S. Pat. No. 6,238,473.
Wiggins, Sr., Crabb & Wiggins, Jr. U.S. Pat. No. 6,761,763 describes a process for compacting calcined kaolin after packaging.
Consequently, there is a need in the industry for a process by which powders (such as calcined kaolin clay, silica powders and air floated clays) can be compacted to provide a product which has an increased bulk density and which can be loaded directly into conventional bulk rail freight cars, bulk highway trucks or bags without the addition of water or other additives.